Self-attaching female fastener elements including nuts having a re-entrant groove are presently formed by rolling a continuous metal strip having the cross-section of the female fastener elements including a continuous projecting pilot portion and flange portions on opposed sides of the pilot portion as disclosed, for example, in U.S. Pat. Nos. 3,187,796 and 3,648,747 assigned to the assignee of the predecessor in interest of the assignee of the present application. The rolled strip is then pierced to form a cylindrical bore and the rolled strip is then severed or chopped, forming discreet female fastener elements or nuts each having a rectangular pilot portion and rectangular flange portions on opposed sides of the pilot portion. As disclosed in the above-referenced U.S. patents, grooves are rolled into either the side portions of the pilot portion or the flange portions on opposed sides of the pilot portion for attachment of the self-attaching female fastener element or nut to a panel. Self-attaching female fastener elements may be utilized to pierce an opening in the panel, commonly referred to as pierce nuts, wherein the rectangular pilot portion pierces the panel and the panel metal adjacent the pierce panel opening is then deformed into the grooves by a die member or die button. Alternatively, the self-attaching female fastener elements may be installed in a pre-pierced opening in the panel. In a typical application, the self-attaching female fastener elements are received in an installation head in an upper die shoe or platen of a die press having a reciprocating plunger and the panel is received on a die button in the lower die shoe or platen, such that a self-attaching female fastener element is installed in the panel with each stroke of the die press.
In one preferred embodiment as disclosed in the above-referenced U.S. Pat. No. 3,648,747, re-entrant grooves are provided in the flange portions on opposed sides of the pilot portion providing improved push-off strength of the self-attaching female fastener element following installation in a panel. As used herein, the term “re-entrant groove” refers to a groove having an opening at the end face of the flange portion having a width less than the bottom wall of the groove, wherein the die button includes projecting lips which deform the panel metal adjacent the panel opening against the bottom wall of the groove, deforming the panel metal beneath the restricted opening to the groove and providing improved retention. In a preferred embodiment, the inner and outer groove walls are inclined toward each other forming a dovetail-shaped groove providing further improved retention of the female fastener element in a panel. At present, however, dovetail-shaped grooves can only be formed by rolling, as described in the above-referenced U.S. patents, which also requires that the pilot and flange portions be rectangular using present technology. However, the rectangular pilot portion may form stress risers at the corners of the rectangular opening which receives the pilot portion of the self-attaching female fastener element as described above. Further, there are many applications which require a self-attaching female fastener element which can presently be formed only by cold forming, including cold heading and progressive dies, such as self-attaching female fastener elements having an annular or cylindrical pilot portion and an annular flange surrounding the pilot portion.
The cold forming method of this invention may be utilized to form self-attaching fasteners having an annular or cylindrical projecting central pilot portion, an annular flange portion surrounding the pilot portion and a re-entrant groove in the flange portion which is one of the objects of this invention. The self-attaching fastener formed by the method of this invention may be utilized either as a pierce nut or clinch nut as described above.